Transmitted data always have the possibility of being corrupted. we would like to be able to detect errors and, better yet, correct them. one approach is to use redundant data, for example, submit three versions of the data so that if one version is corrupted the data can be retrieved from one of the two remaining versions. the downside of this approach is that in many situations the probability of an error is so low that it is a waste of resources to have this much redundancy